package com.ronsoft.books.nio.channels;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.DatagramChannel;
import java.util.Date;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

/**
 * Request time service, per RFC 868. RFC 868
 * (http://www.ietf.org/rfc/rfc0868.txt) is a very simple time protocol whereby
 * one system can request the current time from another system. Most Linux, BSD
 * and Solaris systems provide RFC 868 time service on port 37. This simple
 * program will inter-operate with those. The National Institute of Standards
 * and Technology (NIST) operates a public time server at time.nist.gov.
 * 
 * The RFC 868 protocol specifies a 32 bit unsigned value be sent, representing
 * the number of seconds since Jan 1, 1900. The Java epoch begins on Jan 1, 1970
 * (same as unix) so an adjustment is made by adding or subtracting
 * 2,208,988,800 as appropriate. To avoid shifting and masking, a four-byte
 * slice of an eight-byte buffer is used to send/recieve. But getLong() is done
 * on the full eight bytes to get a long value.
 * 
 * When run, this program will issue time requests to each hostname given on the
 * command line, then enter a loop to receive packets. Note that some requests
 * or replies may be lost, which means this code could block forever.
 * 
 * @author Ron Hitchens (ron@ronsoft.com)
 */
public class TimeClient {
    private static final int DEFAULT_TIME_PORT = 37;
    private static final long DIFF_1900 = 2208988800L;

    protected int port = DEFAULT_TIME_PORT;
    protected List<InetSocketAddress> remoteHosts;
    protected DatagramChannel channel;

    public TimeClient(String[] argv) throws Exception {
        if (argv.length == 0) {
            throw new Exception("Usage: [ -p port ] host ...");
        }

        parseArgs(argv);

        this.channel = DatagramChannel.open();
    }

    protected InetSocketAddress receivePacket(DatagramChannel channel, ByteBuffer buffer) throws Exception {
        buffer.clear();

        // Receive an unsigned 32-bit, big-endian value
        return ((InetSocketAddress) channel.receive(buffer));
    }

    // Send time requests to all the supplied hosts
    protected void sendRequests() throws Exception {
        ByteBuffer buffer = ByteBuffer.allocate(1);
        Iterator<InetSocketAddress> it = remoteHosts.iterator();

        while (it.hasNext()) {
            InetSocketAddress sa = (InetSocketAddress) it.next();

            System.out.println("Requesting time from " + sa.getHostName() + ":" + sa.getPort());

            // Make it empty (see RFC868)
            buffer.clear().flip();
            // Fire and forget
            channel.send(buffer, sa);
        }
    }

    // Receive any replies that arrive
    public void getReplies() throws Exception {
        // Allocate a buffer to hold a long value
        ByteBuffer longBuffer = ByteBuffer.allocate(8);

        // Assure big-endian (network) byte order
        longBuffer.order(ByteOrder.BIG_ENDIAN);
        // Zero the whole buffer to be sure
        longBuffer.putLong(0, 0);
        // Position to first byte of the low-order 32 bits
        longBuffer.position(4);

        // Slice the buffer; gives view of the low-order 32 bits
        ByteBuffer buffer = longBuffer.slice();
        int expect = remoteHosts.size();
        int replies = 0;

        System.out.println("");
        System.out.println("Waiting for replies...");

        while (true) {
            InetSocketAddress sa;

            sa = receivePacket(channel, buffer);

            buffer.flip();
            replies++;

            printTime(longBuffer.getLong(0), sa);

            if (replies == expect) {
                System.out.println("All packets answered");

                break;
            }
            // Some replies haven't shown up yet
            System.out.println("Received " + replies + " of " + expect + " replies");
        }
    }

    // Print info about a received time reply
    protected void printTime(long remote1900, InetSocketAddress sa) {
        // local time as seconds since Jan 1, 1970
        long local = System.currentTimeMillis() / 1000;
        // remote time as seconds since Jan 1, 1970
        long remote = remote1900 - DIFF_1900;
        Date remoteDate = new Date(remote * 1000);
        Date localDate = new Date(local * 1000);
        long skew = remote - local;

        System.out.println("Reply from " + sa.getHostName() + ":" + sa.getPort());
        System.out.println("  there: " + remoteDate);
        System.out.println("   here: " + localDate);
        System.out.print("   skew: ");

        if (skew == 0) {
            System.out.println("none");
        } else if (skew > 0) {
            System.out.println(skew + " seconds ahead");
        } else {
            System.out.println((-skew) + " seconds behind");
        }
    }

    protected void parseArgs(String[] argv) {
        remoteHosts = new LinkedList<InetSocketAddress>();

        for (int i = 0; i < argv.length; i++) {
            String arg = argv[i];

            // Send client requests to the given port
            if (arg.equals("-p")) {
                i++;
                this.port = Integer.parseInt(argv[i]);
                continue;
            }

            // Create an address object for the hostname
            InetSocketAddress sa = new InetSocketAddress(arg, port);

            // Validate that it has an address
            if (sa.getAddress() == null) {
                System.out.println("Cannot resolve address: " + arg);

                continue;
            }
            remoteHosts.add(sa);
        }
    }

    // --------------------------------------------------------------

    public static void main(String[] argv) throws Exception {
        TimeClient client = new TimeClient(argv);

        client.sendRequests();
        client.getReplies();
    }
}
